WEDNESDAY, Jan. 23, 2019 (HealthDay News) -- A pair of twin toddlers who were once joined at the head are "thriving" more than a year after surgeons used new techniques to separate them.

"We are so grateful and feel so blessed that we get to be their parents and watch them grow and thrive," said the twins' father, Riley Delaney.

The twins' doctors described what it took to separate the girls, who shared brain tissue and a critical blood vessel, in a report in the Jan. 24 issue of the New England Journal of Medicine.

The girls -- Erin and Abby -- underwent the separation surgery at Children's Hospital of Philadelphia in June 2017. They were 10 months old. But the process leading up to that 11-hour procedure began when the girls were just 3 months old.

Conjoined twinning is a rare occurrence -- with fusion at the head being rarer still. It happens in an estimated six out of every 10 million births, according to plastic surgeon Dr. Jesse Taylor, a member of the 30-person team who worked on the Delaney twins' case.

It was back in 1957 when doctors reported the first successful separation of twins conjoined at the head -- meaning that both twins survived long term. But for most of the years since, only modest advances have been made in those separation techniques, Taylor said.

That's because the phenomenon is so rare, a surgical team may only ever see one or two cases.

"And surgery is like most things in life," Taylor said. "The more you do it, the better you get."

More recently, though, technology has allowed doctors to generate computer-aided three-dimensional models of the babies' skulls and the anatomical structures inside. In the case of Erin and Abby, that included a vital blood vessel that they shared. Called the superior sagittal sinus, it carries blood from the brain to the heart.

The 3D model, Taylor explained, allowed the surgical team to practice ahead of time.

"That was critically important," he said. "It enabled us to practice with the whole team -- what part of the dance is happening when."

That computer-aided technology has been used in other recent separation surgeries, Taylor said. What's new in this case is what happened in the months before the babies' surgery.

"We drew on our experience in craniofacial surgery," he explained.

Specifically, they turned to a minimally invasive technique called distraction. When the twins were about 3 months old, surgeons cut through the bone where the babies' skulls were joined and then attached a custom-designed external device that performed the distraction: Over a few weeks, it gradually pushed the twins apart by about 2 millimeters per day.

After that, the girls were outfitted with another external device that performed a "constriction." Over two months, it reduced the amount of skull surface area that the babies shared.

During the final, 11-hour procedure, surgeons painstakingly separated the girls, which included dividing the sagittal sinus vessel between them. The babies were 10 months old -- making them among the youngest to ever have such separation surgery, Taylor said.

And based on what's known about the human brain, the earlier the separation can be done, the better.

"That has to do with the brain's plasticity," Taylor explained, referring to the brain's ability to reorganize itself by making new connections among cells.

In infancy, Taylor said, the brain is naturally forging lots of new connections.

After surgery, the girls remained in the hospital for two months before being discharged to inpatient rehabilitation for another couple of months. They returned to their home in Mooresville, N.C., just before Thanksgiving 2017.

"We didn't have surgical complications, there were no infections," Taylor said. And remarkably, he noted, the girls did not develop fluid buildup in the brain, which is typical.

Today, at the age of two-and-a-half, the girls are "exceeding all expectations," according to their parents. Erin is crawling, while Abby is sitting up and getting around her own way.

"She [Abby] rolls around the house like it's her job and gets into everything," the twins' mother, Heather Delaney, said.

Meanwhile, "Erin is crawling everywhere and enjoying her newfound freedom … she is so engaged and curious," Heather added.

A team of specialists will keep following the girls' development, Taylor said. And around age 4 or 5, they will need a synthetic implant to cover the openings in their skulls that remain post-surgery.

More broadly, Taylor said, his team hopes their pre-surgery approach can help others who perform similar separation procedures.